I experienced that once where the tank was stuck in the stadium wall. We were able to kill him by shooting at the wall. Is the elevator poltergeist where the doors wont open and you are trapped? Seen that once too.

The funny thing about this is that the tanks dying is due to a fix patch for versus that they added a few months ago. Never had an issue until the "versus balance patch" that added blindspots for people to spawn in but completely screwed the Campaign players because the tank would die from not being seen. Another bug that was added due to the rebalance patch was the helicopter will fly through the added wall at the end if the copter lands on the left.